1. Field of the Invention
The present invention relates to a nitride semiconductor device which uses a nitride semiconductor (InXAlYGa1-X-YN, 0xe2x89xa6X, 0xe2x89xa6Y, X+Yxe2x89xa61) used in light emitting devices such as light emitting diode device (LED) and laser diode device (LD), light receiving devices such as solar cell and optical sensor or electronic devices such as transistor and power devices, and particularly to a nitride semiconductor device comprising nitride semiconductor layer which includes In.
2. Description of the Prior Art
Recently semiconductor laser devices which use nitride semiconductor have been receiving increasing demands for the applications in optical disk systems such as DVD which are capable of recording and reproducing a large amount of information with a high density. Accordingly, vigorous research efforts are being made in the field of semiconductor laser device which uses nitride semiconductor. Because of 0the capability to oscillate and emit visible light over a broad spectrum ranging from ultraviolet to red, the nitride semiconductor laser, device is expected to have wide applications such as light sources for laser printer and optical network, as well as the light source for optical disk systems. The applicant of the present invention reported a laser which successfully underwent over ten thousand hours of operation under the conditions of continuous oscillation at a wavelength of 405 nm with output power of 5 mW at the room temperature.
Light emitting devices and light receiving devices which use nitride semiconductor have such a structure as a nitride semiconductor which includes In-is used for the active layer and, accordingly, it is important to form a better active region in the active layer in order to improve the device characteristics.
In the prior art, n-type nitride semiconductors doped with n-type impurities have beer used for the active layer of the nitride semiconductor device. Particularly in the case of a device of quantum well structure, the n-type nitride semiconductors doped with n-type impurities have been used in the well layer and, the barrier layer.
In order for light emitting devices which employ nitride semiconductors to have applications in wide fields, they must be further improved in the device characteristics, particularly in the device lifetime.
It is essential to have a longer lifetime and a higher output power for the laser devices which use nitride semiconductors in order to be used as the light source for reading or writing information in high-density optical disk systems described above and have further applications. Other classes of the nitride semiconductor device are also required to have a longer lifetime and a higher output power, and light emitting devices are required to have a higher output power of light emission.
Weak reverse withstanding voltage of the devices using nitride semiconductor, which has been a problem in the part art, has a high probability of leading to destruction of the device during handling in the manufacturing process and mounting on an end product, and is therefore one of the most important problems.
The present invention has been made in consideration of the problems described above, and aims at obtaining a nitride semiconductor device which has excellent device characteristics including the threshold current density and has longer device lifetime and high output power.
(1) A light emitting device according to the present invention is a type of nitride semiconductor device having a structure where an active layer of a quantum well structure, which comprises a well layer made of a nitride semiconductor that includes In, and a barrier layer made of a nitride semiconductor, is sandwiched by a p-type nitride semiconductor layer and an n-type nitride semiconductor layer, wherein the light emitting device according to the present invention is characterized in that the above active layer has a first barrier layer, that is arranged in a position nearest to the above p-type nitride semiconductor layer, and a second barrier layer, that is different from the first barrier layer, as the above barrier layer and is characterized in that the above first barrier layer does not substantially include an n-type impurity while the above second barrier layer includes an n-type purity. Here, though, barrier layers, other than the first barrier layer and the second barrier layer among the barrier layers in the active layer, are not particularly limited, in the case of usage as a laser device or as a light emitting device of high power, they are preferably doped with an n-type impurity or are not doped with any impurities.
Though, in a conventional multiple quantum well-type (hereinafter referred to as MQW-type) nitride semiconductor device, all the barrier layers are, in general, doped with an n-type impurity, such as Si, in order to enhance light emission efficiency by increasing the initial electron concentration in the active layer, a nitride semiconductor device of the present invention has a barrier layer, that is doped with an n-type impurity in the same manner as in the prior art, while an n-type impurity is not substantially included only in the first barrier layer that is nearest to the p-type nitride semiconductor layer. In such a structure, characteristics with respect to the device lifetime and the reverse withstanding voltage of the nitride semiconductor device can be improved.
Though the mechanism where the lifetime characteristic is improved is not necessarily evident, it can be inferred that, for one reason, the fact that the lifetime of the carriers has become longer than in the prior art contributes to this mechanism. Conventionally a barrier layer, that is doped with an n-type impurity, is arranged on the side of the p-type layer so that diffusion of the p-type impurity from the p-type layer occurs to quite a great degree and, thereby, a barrier layer, that includes an n-type impurity and a p-type impurity, is provided, which causes the lowering of the lifetime of the carriers. According to the present invention, since the first barrier layer is not doped with an n-type impurity, n-type and p-type impurities can be prevented from coexisting in the same barrier layer.
In addition, among barrier layers in the active layer, the barrier layer arranged on the side of the p-type layer (first barrier layer) does not substantially include an n-type impurity so as to have a function different from that of the barrier layer (second barrier layer), which has an n-type impurity, in the active layer. That is to say, by having the second barrier layer, the carriers injected from the n-type layer into the active layer are increased and the carriers that reach deep into the active layer (to the p-type layer side) are increased so that the injection efficiency of the carriers can be increased while, by having the first barrier layer, a barrier layer, in which an n-type impurity is not included, is arranged as a barrier layer nearest to the p-type layer in the active layer so that it becomes possible to increase the injection of the carriers from the p-type layer and also to improve the efficiency.
In the case that an n-type impurity is included in the first barrier layer, the injection of the carriers from the p-type layer tends to be blocked. In particular, the diffusion distance of the carriers from the p-type layer tends to be short in comparison with the carriers from the n-type layer and, therefore, when the first barrier layer, which corresponds to the entrance for the injection of the carriers from the p-type layer to the active layer, has an n-type impurity, the injection of the carriers from the p-type layer is negatively affected to a serious degree. As shown in FIG. 14, it is understood that the device lifetime is suddenly lowered as the n-type impurity concentration in the first barrier layer is increased.
Accordingly, by providing the first barrier layer in the active layer, it is observed that a great number of holes can be provided and the lifetime of the carriers tends to become longer such that they are considered to contribute to the increase of the above characteristics.
Though the second barrier layer may adjoin the first barrier layer, it is preferably provided at a distance away from the first barrier layer by making at least one, or more, well layer intervene. Thereby, the first barrier layer arranged on the p side and the second barrier layer arranged on the n side are provided with a well layer placed between them within the active layer so that an effective carrier injection becomes possible so as to reduce the loss in a laser device as a light source for, for example an optical disk system, and the device characteristics, in particular the device lifetime and the power, are subsequently increased. At this time, the second barrier layer is preferably a barrier layer nearest to the n-type layer among the barrier layers in the active layer so as to be the entrance for the injection of the carriers from the n-type layer so that a great amount of carrier injection or an effective injection becomes possible and the device characteristics are improved.
Here, the fact that an n-type impurity is not substantially included indicates that an n-type impurity is not included due to the exceeding of the concentration resulting from the contamination, or the like, during the process and, for example, in the case that the n-type impurity is Si, the fact indicates that the concentration is 5xc3x971016 cmxe2x88x923, or less.
(2) It is preferable for the film thickness of the above first barrier layer to be greater than the film thickness of the second barrier layer. In this structure increase of the device lifetime can be implemented. In the case that the first barrier layer has a film thickness less than that of the other barrier layer (second barrier layer), lowering of the device lifetime can be observed. In particular, this tendency is significant in the case that the first barrier layer is arranged in the outermost position. In addition, in the case that the first barrier layer is positioned in the outermost position in the active layer, that is to say, on the top, when a p-type nitride semiconductor layer is provided on the active layer, the reduction of the above device lifetime is furthered. For example, as shown in FIG. 8, in the case that the first harrier layer 2c is arranged nearest to the p-type electron confining layer (first p-type nitride semiconductor layer), the first barrier layer becomes an important layer where the film Thickness thereof determines the characteristics of the active layer and the well layer since this p-type electron confining layer is a layer that strongly affects the active layer, particularly the well layer, as described below.
That is to say, in the nitride semiconductor device according to the present invention, carriers can be effectively confined in the active layer when the barrier layers in the active layer are formed of a nitride semiconductor that includes In and the layer, at least, adjoining the active layer among p-type nitride semiconductor layers is formed of a nitride semiconductor (electrons confining layer) that includes Al. However, when a nitride semiconductor that includes Al is made to grow after a nitride semiconductor that includes In is made to grow, the nitride semiconductor that includes In is easily resolved because of the high vapor pressure of InN and because of the difference in the growth conditions of these nitride semiconductors. Therefore, it is preferable for the first barrier layer to be formed thicker than the other barrier layers.
For example, in the case of the growth by means of an MCCVD method, it is general that InGaN is made to grow under the conditions of a slow gas flow rate at a low temperature in a nitrogen gas atmosphere while AlGaN is made to grow under the conditions of a fast gas flow rate at a high temperature in a hydrogen gas atmosphere. Accordingly, for example, when, after growing InGaN as, the first barrier layer, AlGaN is made to grow as a p-type nitride semiconductor layer, InGaN is resolved through a gas etching at the time when the growth condition within the reaction vessel is switched to another condition. Therefore, by forming the first barrier layer thicker than the other barrier layers, an excellent quantum well structure can be maintained even in the case that the first barrier layer is slightly resolved. That is to say, the first barrier layer plays the role of a protective layer that prevents the active layer, which includes In, from being resolved.
Furthermore, in the case that the first barrier layer arranged nearest to the p-type nitride semiconductor layer has a great film thickness, the distance vis-à-vis the p-type electron confining layer can be increased so that carriers of a high concentration can be stably injected in the continuous drive of the device since a sufficiently broad space can be secured even for a great amount of p-type carriers. Accordingly, device reliability, such as a long device lifetime, can be improved.
(3) In addition, when the barrier layer arranged in the position nearest to the n-type nitride semiconductor layer is assumed to be a barrier layer B1 and the i-th (i=1, 2, 3 . . . L) barrier layer counted from the barrier layer B1 toward the above p-type nitride semiconductor layer is assumed to be a barrier layer B1, it is preferable for barrier layers Bi from i=1 to i=n (1 less than n less than L) to have an n-type impurity. Because of this structure the injection of the carriers to each well layer in the active layer becomes more efficient. In addition, the injection of the carriers deep into the active layer (p-type layer side) is effectively carried out so that the device can deal with a great amount of carrier injection. Accordingly, the light emission efficiency is improved, for example, in an LED or in a LD and it becomes possible to lower the oscillation threshold current density and the forward direction voltage while increasing the device lifetime. In addition, the provision of an n-type impurity in the barrier layers bi from the first to the n-th contributes to the lowering of the threshold current density because the carriers are immediately injected into the well layers at the initial phase of the drive of the device.
(4) In addition, all of the barrier layers, other than the first barrier layer, are preferably doped with an n-type impurity. Thereby, the carrier injection from the n-type layers can further be increased and can be made more effective.
(5) It is preferable for the above first barrier layer to be arranged in the outermost position of the above active layer. The first barrier layer is arranged on the side nearest to the p-type nitride semiconductor layer within the active layer so that the first barrier layer becomes the entrance for the injection of the carriers and, thereby, the injection of the carriers from the p-type layer to the active layer becomes effective and a great amount of carriers can be injected so as to improve device characteristics, such as the threshold current density, the device lifetime and power. In addition, a nitride semiconductor device can be gained which has the device reliability that can withstand severe drive conditions, such as a great amount of current or a high power. At this time, it is preferable for the p-type nitride semiconductor layer to be formed so as to contact the active layer and the below described first p-type nitride semiconductor layer can be provided as a layer that contacts the first barrier layer.
(6) Furthermore, it is preferable for the above second barrier layer to be arranged in the outermost position close to the above n-type nitride semiconductor layer within the above active layer. In this structure, the active layer is provided wherein the first p side barrier layer and the second n side barrier layer are respectively arranged on the p-type nitride semiconductor layer side and on the n-type nitride semiconductor layer side so that the carriers from the p-type layer and n-type layer are effectively injected toward the center portion of the active layer.
(7) It is preferable in the above structure (6) for the film thickness of the above first p side barrier layer to be approximately the same as the film thickness of the above second n side barrier layer. In this structure, the active layer becomes more symmetrical and, as a result, the dispersion of the devices can be restrained so as to increase the yield and the threshold current density is reduced.
(8) In addition, it is preferable in the above structure (6) for the above active layer to have two, or more, well layers so as to have a third barrier layer between these well layers and it is also preferable for the film thickness of the above third barrier layer to be less than the film thicknesses of the above first p side barrier layer and of the second n side barrier layer. In this structure, it becomes possible for the second n side barrier layer and the first p side barrier layer, as well as the third barrier layer, to have different functions so that it becomes possible to restrain the dispersion of the device characteristics and to reduce the threshold current density Vf. That is to say, the second n side barrier layer and the first p side barrier layer are arranged in the outermost position in the active layer so as to be the entrances for the injections of the carriers from the n-type layer and p-type layer while the film thickness is greater than the third barrier layer so that a broad space for holding a great amount of carriers is secured and, contrarily, the film thickness of the third barrier layer is small so that the film thickness of the entirety of the active layer can be reduced so as to contribute to the reduction of Vf.
(9) It is preferable for at least one well layer within the above active layer to have a film thickness of 40 xc3x85, or more. Conventionally, the film thickness of the well layer is regarded as optimal in a preferable range of from approximately 20 xc3x85 to 30 xc3x85 since the characteristics (for example, oscillation threshold current) at the initial stage of oscillation and the light emission are taken into consideration which results in the fact that a continuous drive with a great current accelerates the device deterioration and prevents the increase of the device lifetime. The present invention solves this problem due to the above structure.
That is to say, the structure of the present invention makes an effective carrier injection possible and, additionally, by providing a well layer of which the film thickness is suitable for the carrier injection, it becomes possible to increase the stability in the drive of a light emitting device and a laser device of high power and loss in output, relative to the injected current, can be reduced so that a great increase in the device lifetime can be made possible. An effective light emitting recombination, without loss, of the great amount of carriers injected in the well layer is required for light emission and oscillation at high power and the above structure is suitable for implementing such light emission and oscillation.
The upper limit of the film thickness of the well layer depends on the film thicknesses of the barrier layers and of the active layer and is preferably 500 xc3x85, or less, though it is not particularly limited to this. In particular, it is preferably 300 xc3x85, or less, when it is taken into consideration that a plurality of layers are layered in the quantum well structure. Furthermore, in the case that the film thickness of a well layer is in the range of no less than 50 xc3x85 and no more than 200 xc3x85, it is possible to form, preferably, an active layer in either a multiple quantum well structure or in a single quantum well structure. In the case of the multiple quantum well structure in particular, the film thickness of a well layer is preferably in the range of no less than 50 xc3x85 and no more than 200 xc3x85, since the number of layers (number of pairs of a well layer and a barrier layer) is increased. In addition, when the film thickness of a well layer is in this preferable range, a high reliability of the device and a long lifetime can be gained for light emission and oscillation with a large amount of current and with a high power output while, in a laser device, a continuous oscillation at 80 mW becomes possible and an excellent device lifetime can be implemented in a broad output range such as from 5 mW to 80 mW. At this time, it is necessary to adopt the above range of film thickness of a well layer for at least one well layer in the case that the active layer has a multiple quantum well structure and preferably the above film thicknesses are adopted for all of the well layers. By doing so, the above described effects are gained in each of the well layers so that a light emitting recombination and a photoelectric conversion efficiency are further improved. By using a nitride semiconductor that includes In, more preferably InGaN, for a well layer, an excellent device lifetime can be gained. At this time, by making the composition ratio x of In in the range of 0 less than xxe2x89xa60.3, a well layer of a thick film with a good crystal can be formed and preferably by making xxe2x89xa60.2, a plurality of well layers of thick films with a good crystal structure can be formed so that an active layer in a good MQW structure can be gained.
(10) The above described first barrier layer preferably has a p-type impurity. In this structure, the above described injection of carriers from the p-type layer becomes effective and the lifetime of the carriers tends to increase and, as a result, the structure contributes to increases in the reverse withstanding voltage, the device lifetime and the output. This is because, as described above, a carrier injection from the p-type layer becomes excellent due to substantially no inclusion of an n-type impurity and, furthermore, it becomes possible to accelerate further injection of carriers into the active layer by having a p-type impurity in the first barrier Layer so that a large amount of carriers are effectively injected from the p-type layer into the active layer or deep inside the active layer (n-type layer side) and, thereby, increases in light emitting recombination, photoelectric conversion efficiency and device lifetime and, in addition, an improvement in the characteristic of reverse withstanding can be implemented.
(11) In addition, though the concentration of the p-type impurity in the first barrier layer is not in particular limited, it is preferable to be no less than 1xc3x971016 cmxe2x88x923 and no more than 1xc3x971019 cmxe2x88x923. In the case the p-type impurity concentration is too low, the hole injection efficiency into the well layer is lowered, while if it is too high, the carrier mobility in the first barrier layer is lowered so as to increase the Vf value of the laser.
(12) The first barrier layer of which the p-type impurity concentration is in such a range is an i-type or a p-type.
(13) The doping of a p-type impurity into the first barrier layer is preferably carried out through diffusion from the p-type nitride semiconductor layer after making the undoped first barrier layer grow rather than being carried out at the time of the growth of the first barrier layer. This is because, in the case that it is carried out at the time of the growth of the first barrier layer, a p-type impurity diffuses into an n-type well layer beneath the first barrier layer at the time when the first barrier layer grows so that the device lifetime characteristics is lowered, on the other hand, in the case that the doping of a p-type impurity is carried out through diffusion, a p-type impurity can be doped into the first barrier layer without affecting the well layer.
(14) In the case that an n-type nitride semiconductor layer, an active layer and a p-type nitride semiconductor layer are layered in sequence in the device structure, the barrier layer can have a p-type impurity because a p-type impurity diffuses from the p-type nitride semiconductor layer that is made to grow subsequent to the first barrier layer, which is made to grow without doping.
(15) A nitride semiconductor device of the present invention preferably has a laser device structure wherein said p-type nitride semiconductor layer has an upper clad layer made of a nitride semiconductor that includes Al of which the average mixed crystal ratio of x, wherein 0 less than xxe2x89xa60.05 and wherein said n type nitride semiconductor layer has a lower clad layer made of a nitride semiconductor that includes Al of which the average mixed crystal ratio of x, wherein 0 less than xxe2x89xa60.05. A laser device gained in such a structure can continuously oscillate with the output of 5 mW to 100 mW so as to become an LD having device characteristics suitable for a reading and writing light source in an optical disk system and makes it possible to implement a long lifetime. By limiting the average mixed crystal ratio of Al in the clad layer to 0.05, or less, an optical wave guide which makes it possible to control a self-exciting oscillation at the time of a high power output is provided so that a continuous oscillation with a high power output in a stable manner becomes possible and it also becomes possible to gain an LD for an optical disk light source. Though, conventionally, a nitride semiconductor of which the average composition of Al in the clad layer is no less than 0.05 and no more than 0.3 is used, in this structure confinement of light becomes too strong and, thereby, a self-exciting oscillation is generated in a continuous oscillation with a high output of 30 mW, or more. This self-exciting oscillation is due to the generation of a kink in the current-light output characteristics that is generated on the low output side in an LD structure which has a strong light confinement in the longitudinal direction so as to enhance the light density and such generation of a kink thus becomes disadvantageous as a light source of an optical disk system so that a self-exciting oscillation due to the kink is unstable and leads to dispersion in the devices. According to the structure of the present invention, an optical wave guide, of which the refraction difference in a clad layer is reduced, is gained and by using an active layer that is in the above described range, a large amount of carriers are continuously injected in a stable manner for light emitting recombination in the structure so that a continuous oscillation can be gained so as to exceed the compensation for the loss due to the lowering of the light confinement in the clad layer and, moreover, the light emission efficiency within the active layer can be enhanced.
It is preferable that said upper clad layer has a p-type conductivity and said lower clad layer has an n-type conductivity, and that said active layer has a first barrier layer that is arranged in a position nearest to said upper clad layer as said barrier layer and a second barrier layer that is different from the first barrier layer and, at the same time, it is preferable that said first barrier layer has a p-type impurity and said second barrier layer has an n-type impurity. In such a structure, as described above, injection of carriers from the p-type layer is carried out in an excellent condition and, as a result, device characteristics, in particular device lifetime, are improved.
(16) The above p-type nitride semiconductor layer preferably contains a first p-type nitride semiconductor layer so as to adjoin the active layer so that the first p-type nitride semiconductor layer is made of a nitride semiconductor that includes Al. In this structure, as shown in FIGS. 4 to 7, the first p-type nitride semiconductor layer 28 functions as an electron confining layer and, in particular, makes it possible to confine a large amount of carriers within the active layer in an LD and an LE with a large current drive for high power output. In addition, in the relationships between the above first barrier layer, barrier layer BL and the first p side barrier layer, as shown in FIG. 8, the film thicknesses of these barrier layers determine the distance dB between the first p-type nitride semiconductor layer and a well layer 1b so as to greatly affect the device characteristics.
In addition, since the first p-type nitride semiconductor layer may grow in the form of a thin film, it can be made to grow at a temperature lower than that for a p-type clad layer. Accordingly, by forming a p-type electron confining layer, resolution of an active layer that includes In can be prevented as opposed to the case where a p-type clad layer is directly formed on the active layer. That is to say, the p-type electron confining layer plays the roles of preventing the resolution of the active that includes In in the same manner as does the barrier layer of FIG. 1.
(17) The above described first p-type nitride semiconductor layer is provided so as to contact the barrier layer nearest to the above described p-type nitride semiconductor layer, and, preferably, is formed of a semiconductor which has been grown by being doped with a p-type impurity of which the concentration is higher than that in a barrier layer in the above active layer. Because of this structure, the injection of carriers from the p-type layer to the barrier layer (the above described first barrier layer) closest to the p-type layer becomes easy to implement. In addition, by doping a p-type impurity of a high concentration into the first p-type nitride oxide layer, a p-type impurity is diffused into the barrier layer so that an appropriate p-type impurity can be added. Thus, since it becomes unnecessary to add an impurity at the time of barrier layer growth, it becomes possible to make a barrier layer grow with a good crystal structure. In particular, in the case that this barrier layer is a nitride semiconductor that includes In, crystal deterioration is great due to impurity addition and the effect thereof is significant. In addition, in the case that the first p-type nitride semiconductor layer is, as described below, a nitride semiconductor that includes Al and the Al mixed crystal ratio thereof is higher than the mixed crystal ratio of Al in the p-type clad layer, it effectively functions as an electron confining layer which confines electrons within the active layer and the effects are gained wherein an oscillation threshold value and driving current are lowered in a large current drive, a high power LD and an LED.
(18) It is preferable for the number of well layers to be in the range of from no less than 1 to no more than 3 in the above described active layers. In this structure it becomes possible to lower the oscillation threshold value in comparison with the case where the number of threshold layers is 4, or more. In addition, at this time by making the film thickness of the well layer 40 xc3x85, or more, as described above, a broad space is secured inside of a small number of well layers so that an effective light emitting recombination becomes possible even in the case that a large amount of carriers are injected so that this makes an increase in the device lifetime and an increase in the light emission output possible. In particular, in the case that the film thickness of the well layer is 40 xc3x85, or less, and the number of well layers is 4, or more, a large amount of carriers are infected into each well layer of a thin film, in comparison with the above described case, in order to gain a high power LD or LED by driving the well layers with a large current so that the well layers are driven under severe conditions and device deterioration occurs at an early stage. In addition, when the number of well layers increases, the carriers are not distributed uniformly but, rather, tend to be distributed unevenly so that the above described device deterioration becomes a critical problem in the case wherein the device is driven with a large amount of current under such a condition. In such a structure as described above, the barrier layer that is nearest to the p-type layer side does not include an n-type impurity or have a p-type impurity, or other barrier layers each do have an n-type impurity and, thereby, a large amount of carriers can be injected into the well layers in a stable manner and, in addition, the well layer is maintained at the above described film thickness (40 xc3x85, or more) so that these closely relate to each other to appropriately work to implement an excellent device lifetime and a high light emission output in a consecutive driving of the device.
(19) It is preferable that the above described second barrier layer is arranged so as to be sandwiched by well layers and, the film thickness ratio Rt of the above described well layer to the second barrier layer is in the range of from 0.5xe2x89xa6Rtxe2x89xa63 Because of this structure a light emitting device and a laser device can be gained wherein the response characteristics are excellent and RIN is low in order to be used specifically for an optical disk system, an optical communication system, and the like. That is to say, though the film thicknesses of the well layers, the barrier layers and the active layers become factors that greatly affect the RIN and the response characteristics in the active layer of a quantum well structure, a light emitting device and a laser device that are excellent in these characteristics can be gained by limiting the film thickness ratio of the well layer to the barrier layer to the above described range in this structure.
(20) It is preferable for the film thickness dw of the above described well layer to be in the range of 40 xc3x85xe2x89xa6dwxe2x89xa6100 xc3x85 and for the film thickness db of the above described second barrier layer to be in the range of dbxe2x89xa740 xc3x85. In this structure, by adjusting the film thickness of the well layer so that the above described film thickness ratio Rt is in the above described range, a laser device having a long lifetime, a high power output, as shown in FIG. 12, and having RIN characteristics as well as response characteristics that are suitable for the light source of an optical disk system. That is to say, in the light emitting device of the present invention the lifetime can be made longer by increasing the film thickness of the well layer while, on the other hand, the response characteristics and the RIN characteristics tend to be lowered when the film thickness of the well layer is increased. In this structure, this is appropriately improved. In addition, in the case that the film thickness of the barrier layer is 40 xc3x85, or more, an excellent device lifetime can be gained so that a laser device that becomes an-excellent light source in an optical disk system can be gained as shown in FIG. 13.
(21) It is preferable for the above described p-type nitride semiconductor layer and the above described n-type nitride semiconductor layer to have, respectively, an upper clad layer and a lower clad layer so that the average mixed crystal ratio of Al in the nitride semiconductor of the upper clad layers become greater than that in the lower clad layers. This is because, in an effective refraction type laser device, confinement in the lateral direction can be reduced by increasing the mixed crystal ratio of Al in the upper clad layer, wherein an effective refraction difference is created, so as to have an upper clad layer of which the refraction is small. That is to say, by reducing the refraction difference between a buried layer, which creates an effective refraction difference on both sides of the wave guide, and the upper clad layer, the structure can be gained wherein the confinement in the lateral direction is made smaller. Thus, the confinement in the lateral direction is reduced and light density is reduced and, thereby, a laser device can be gained wherein, up to a high output range, no kink is generated.
(22) Furthermore, the average mixed crystal ratio x of Al in the above described upper clad layer is in the range of 0 less than xxe2x89xa60.1 and, thereby, a laser device can be gained which has the laser characteristics, in particular, characteristics such as current-light output characteristics, that can be used appropriately for an optical disk system. At this time the oscillation wavelength of the laser device can be adjusted in the range of from no less than 380 nm to no more than 420 nm so that an appropriate laser device can be gained by using the above described clad layer.
(23) The above described p-type nitride semiconductor layer has a first p-type nitride semiconductor layer that contacts the above described active layer and becomes an electron confining layer and the active layer has a well layer of which the distance dB from the first p-type nitride semiconductor layer is in the range of from no less than 100 xc3x85 to no more than 400 xc3x85 and has a first barrier layer within the distance dB and, thereby, a nitride semiconductor device of which the device lifetime is excellent can be gained. This can be made to be a device structure wherein, as shown in FIG. 8, the distance dB from the first p-type nitride semiconductor layer 28 has a first barrier layer, that is to say, a barrier layer that does not substantially have an n-type impurity or that is adjusted to have a p-type impurity and, thereby, device deterioration due to the first p-type nitride semiconductor layer, which is a p-type carrier confining layer, is prevented so as to improve the device lifetime and it becomes possible to accelerate a light emitting recombination in a well layer arranged outside of the distance dB. Here, the device has, at least, the above described first barrier layer in the region of the distance dB, that is to say, the device has an impurity adjusted region, wherein, as described above, an impurity, or the amount of the impurity, is adjusted in at least a portion thereof in the region of the distance dB. At this time, the distance dB is preferably the first harrier layer, that is to say, a first barrier layer of which the film thickness is dB is preferably formed so as to contact the first p-type nitride semiconductor layer so that the above described effects can be maximally gained. In this manner, by using the region of the distance dB as an impurity adjusted region, as shown in FIG. 8B, a structure can be gained wherein a plurality of layers of different band gap energies are provided. For example, in FIG. 8B a region 4, of which the band gap energy is smaller than that of the barrier layer 2c, is formed wherein the above described effects can be gained by making the region dB an impurity adjusted region. Contrarily, in a similar manner, a layer 4, which has band gap energy larger than that of the barrier layer 2b, may be provided. That is to say, in the case that a plurality of layers of which the band gap energies are different are provided in the region dB, a device of which the characteristics are excellent can be gained by adjusting the impurity, or the amount of the impurity, in the region dB which is used as the first barrier layer. Furthermore, the distance dB is preferably in the range of from no less than 120 xc3x85 to no more than 200 xc3x85 so that a nitride semiconductor device with an appropriate active layer in the device structure can be gained.
As the n-type impurity used in the nitride semiconductor device of the present invention, group IV or VI elements such as Si, Ge, Sn, S, O, Ti and Zr may be used, while Si, Ge or Sn is preferable and most preferably Si is used. As the p-type impurity, Be, Zn, Mn, Cr, Mg, Ca or the like may be used, and Mg is preferably used.
For the purpose of the present invention, the term undoped means a nitride semiconductor grown without adding p-type impurity or n-type impurity as a dopant, for example organometallic vapor phase growing process in a reaction vessel without supplying any impurity which would serve as dopant.